Influenza virus infects 5-20% of the population and results in 30,000-50,000 deaths each year in the U.S. Although the influenza vaccine is the primary method of infection prevention, four antiviral drugs are also available in the U.S.: amantadine, rimantadine, oseltamivir and zanamivir. As of December 2005, only oseltamivir (TAMIFLU™) is recommended for treatment of influenza A due to the increasing resistance of the virus to amantadine and rimantidine resulting from an amino acid substitution in the M2 protein of the virus. Recently, a drug resistant avian virus was found in a 14-year-girl in Viet Nam. Resistance to Tamiflu has also been found in human influenza as well (Mai Le et al., Nature 437:1108 (2005)).
Influenza vaccines have been demonstrated to have a protective effect against influenza infection. However, yearly emerging antigenic variants of influenza viruses necessitate surveillance to contemporary of circulating virus strains. In some cases, difficulty in the prediction of new variant strains has prevented the timely production of the vaccine (Frace et al., Vaccine 17:2237 (1999)). Recently, pandemic avian influenza has become a serious threat due to the emergence of avian influenza viruses such as H5N1 in southern Asia. The currently available vaccines would be ineffective against avian viruses (Lipatov et al., J. Virology 78:8951 (2004); Osterholm et al., N Engl. Med. 352:1839 (2005)). A third problem with the current vaccine is the ineffectiveness in certain populations with compromised immune systems, for instance premature infants, the elderly, AIDS and transplant patients.
Disease caused by influenza A viral infections is typified by its cyclical nature. Antigenic drift and shift allow for different A strains to emerge every year. Added to that, the threat of highly pathogenic strains entering into the general population has stressed the need for novel therapies for flu infections.